Assuming that you soldier on during those runs with weirdo tanks, you can always Repose enemies that aren't being attacked to ease your healing burden. If they're actually trying and struggling, stick close to the tank so they have an easier time getting the mobs off of you. One less thing for the tank to focus on when you aren't running around everywhere!

Otherwise, about the only thing you can really do is gently remind them to put on their Tank stances and use their basic AoE. As Larek said, it's pretty difficult to mess up aggro as a tank that's actually putting in the minimum of effort; they have to be trolling at that point, or they aren't even trying. Gear is hardly an excuse either - you'd have to probably be at least an expansion behind compared to the rest of the team for aggro to be an issue for a trying tank.